Proteinase-activated receptors (PARs) are part of a distinct and growing class of G-protein-coupled receptors (GPCRs) with the first PAR, the thrombin receptor, now termed PAR-1, first identified ten years ago (Vu et al. (1991). Cell 64:1057–1068). PAR-1 becomes activated following proteolytic cleavage by the serine proteinase thrombin within the amino-terminus. Substantial evidence indicates that the neo-amino terminus, generated after proteolytic cleavage site is the PAR-1's own tethered ligand. Binding of the tethered ligand to the receptor domain leads to receptor activation. Thus, the PAR class of GPCRs are unique in that part of the polypeptide sequence of the receptor encodes the sequence of the receptor's ligand or activating protein (AP).
Additional PARs have been identified. These include PAR-2 (Nystedt et al. (1994). Proc. Natl. Acad. Sci. USA 9208–9212) and PAR-3 (Ishihara et al. (1997). Nature (London) 386:502–506). Most recently the human (Xu et al. (1998). Proc. Natl. Acad. Sci. U.S.A. 95:6642–6646) and murine cDNAs (Kahn et al. (1998). Nature (London) 394:690–694) for PAR-4 have been identified.
Human platelets aggregate in response to thrombin and this response is mediated through PAR-1. A reduction in PAR-1 results in a decrease in thrombin-induced platelet aggregation. The aggregation of human platelets can be reproduced by the exogenous addition of both human PAR-1 AP (SFLLRN-NH2, SEQ ID No.: 8) and murine PAR-1 AP (sequence identical to human PAR-1 AP), however, rodent platelets respond differently to thrombin. A deficiency in mouse PAR-1 does not reduce thrombin-induced platelet aggregation in the mouse (Connolly et al. (1996). Nature 381:516–519; Darrow et al. (1996). Thromb. Haemostasis 76:860–866) demonstrating that the aggregation of murine platelets is mediated through a different receptor for thrombin. Moreover, human AP won't activate mouse PAR-4 indicating that the mouse model may not be an accurate model for understanding human PAR-4 activation. The primary thrombin receptor in the mouse is now known to be PAR-4 (Kahn et al. (1998) Nature (London) 394:690–694) and this is likely to be the rat receptor as well. Recent evidence indicates that human platelets respond to thrombin through a combination of PAR1 and PAR4 (Kahn et al. (1999). J. Clin. Invest. 103:879–887).
Therefore a need exists to develop a small animal model that more reproducibly mimics human thrombosis and platelet function.